The present invention is described in the German priority application No. 19849441.6 filed Oct. 27, 1998 which is hereby incorporated by reference as is fully disclosed herein.
The present invention relates to water-soluble, sulfoalkyl-containing, hydrophobically modified cellulose ethers, to processes for preparing them and to their use as protective colloids in polymerizations.
The preparation of vinyl polymers by free-radical polymerization in an aqueous, solvent-free medium makes it necessary to emulsify the hydrophobic monomers and, after polymerization is complete, to stabilize the polymer. For this reason, the polymerization of monomer systems comprising water-insoluble vinyl monomers in aqueous systems requires not only surfactants but also protective colloids which, on the one hand, have hydrophilic character and, on the other hand, should also have a dispersing action.
The quality of a polymer dispersion is decisively influenced by the choice of the protective colloid. Important quality criteria which can be influenced by the protective colloids are, for example, the stability, viscosity, rheology, the particle size of the polymer particles of the dispersion, and also the amount of coagulum which remains on filtering the dispersion through a sieve. The molecular weight is also influenced by the protective colloid. A further quality criterion is the water absorption of a film which has been produced by spreading and drying a dispersion. This property too is influenced by the protective colloid. In suspension polymerization, the protective colloid controls the particle size of the polymer formed.
It has been known for a long time that polymeric carbohydrates such as starch, dextrans and water-soluble cellulose derivatives are suitable protective colloids for water-based polymerization systems. The protective colloid used most frequently in the commercial production of polyvinyl acetate dispersions is hydroxyethylcellulose (Cellulose and its Derivatives, chapter 26, Ellis Horwood Limited 1985), which is produced on an industrial scale from cellulose and ethylene oxide.
The decisive process in the use of protective colloids in emulsion polymerization is regarded as being free-radical formation on the protective colloid and subsequent grafting of the monomer onto the colloid. The degree of grafting depends on the choice of free-radical initiator. Free-radical initiators customarily used are diazo compounds, redox initiators, organic and inorganic peroxo compounds. On the other hand, the degree of grafting also depends on the nature of the protective colloid. If the degree of grafting is low, the chosen concentration of the protective colloid must be appropriately high in order to achieve a sufficient effect. However, a high protective colloid concentration is undesirable, on the one hand, for cost reasons and, on the other hand, it also leads to increased hydrophilicity of the filmed polymer, in conjunction with increased water absorption.
In U.S. Pat. No. 4,845,175 it is shown that the use of hydroxyethylcellulose which has been hydrophobically modified with arylalkyl groups enables the amount of protective colloid to be produced. Hydrophobically modified cellulose ethers, however, possess a reduced solubility in water.
U.S. Pat. No. 4,868,238 describes carboxymethyl-bearing, hydrophobically modified cellulose ethers as protective colloids in suspension polymerization. Carboxymethyl groups, however, are sensitive to polyvalent cations as are used as electrolytes.
P. Talaba, I. Srokova, P. Hodul and G. Cik in Chem. Papers 50 (2), 101 (1996) describe hydrophobically modified sulfoethylcelluloses. High degrees of substitution are necessary in these compounds, however, because of the absence of other substituents. They are water-soluble only at low degrees of polymerization, and possess a strong tendency to form foam, which is undesirable for use in emulsion paints. EP-A-0 781 780 describes sulfoalkylated cellulose ethers modified hydrophobically using C10-C40 alkyl chains, these ethers likewise possessing a high surfactant action and being used as thickeners in cosmetic formulations. For these compounds a degree of sulfoalkylation of from 0.1 to 1 is claimed.
The object of the present invention was therefore to develop new kinds of protective colloids for polymerizations in aqueous systems, which ensure the same or better quality of the polymer dispersions or polymer suspensions prepared, with a reduction in the amount used and with good processing properties.
It has been found, surprisingly, that hydrophobically substituted sulfoalkyl-HECs are outstandingly suitable as a protective colloid in emulsion polymerization if the degree of substitution in terms of hydrophobic alkyl groups is greater than or equal to 0.001 but does not exceed 1.0, preferably 0.2, alkyl groups per monomer unit. When protective colloids of this kind are used, the amount required is substantially lower than in the case of conventional HEC protective colloids.
The invention accordingly provides water-soluble ionic cellulose ethers from the group of hydroxyalkylcelluloses which are substituted by on average from 0.001 to 1.0, preferably from 0.001 to 0.2, alkyl group per anhydroglucose unit and carry from 0.01 to 0.1 sulfoalkyl group per anhydroglucose unit.
Preferred cellulose ethers are those of the formula
[C6H7O2(OR1)(OR2)(OR3)]m
where C6H7O2 is an anhydroglucose unit,
m is 50-3000, especially 100-1000,
and R1, R2, R3 independently of one another are each a polyalkylene oxide chain of the formula 
where X=H, CnH2n+1, CnH2n+1O, CH2xe2x80x94CH2xe2x80x94SO3Y or CH2xe2x80x94CHOHxe2x80x94CH2SO3Y,
n=4-20
and Y=H, Na or K,
and in which
p, q, and r independently of one another in R1, R2 and R3 can each independently assume values from 0 to 4, the sum of all (p+q+r) added over R1, R2 and R3 per anhydroglucose unit is on average greater than 1.3 and less than 4.5, preferably from 1.5 to 3.0, the sequence of the oxyalkylene units in the polyalkylene oxide chain is arbitrary, and the average number of hydrophobically modified groups per 10 anhydroglucose unit (DS HM) is from 0.001 to 0.2, preferably from 0.01 to 0.04, and the average number of sulfoalkyl groups per anhydroglucose unit is from 0.01 to 0.1, preferably from 0.01 to 0.09. The sulfoalkyl groups are preferably sulfoethyl groups.
The present invention also provides processes for preparing the cellulose ethers of the invention by etherifying cellulose with an etherifying agent from the group of the alkylene oxides and etherifying with an alkyl halide or an alkyl glycidyl ether and a sulfonate, with base catalysis, or by etherifying ethers from the group of hydroxyalkylcelluloses with an alkyl halide or an alkyl glycidyl ether and a sulfonate, with base catalysis, preferably
A) by etherifying cellulose with ethylene oxide, propylene oxide and/or glycidyl alcohol and an alkyl halide or an alkylene oxide or an alkyl glycidyl ether and an alkenylsulfonate or chloroalkylsulfonate, with base catalysis, preferably in a suspension medium;
B) by etherifying hydroxyethylcellulose, hydroxypropylcellulose, dihydroxypropylcellulose or a cellulose ether having two or more of said hydroxyalkyl substituents with an alkyl halide or an alkylene oxide or an alkyl glycidyl ether and an alkenylsulfonate or chloroalkylsulfonate, with base catalysis, preferably in a suspension medium.
Suspension media preferably used are lower alcohols or ketones, an example being isopropanol, tert-butanol or acetone, in a weight ratio to the cellulose of from 3:1 to 30:1, preferably from 8:1 to 15:1. As the base it is usual to use aqueous solutions of alkali metal hydroxides, especially sodium hydroxide. The molar ratio of base to anhydroglucose unit is determined by the carbohydrate (derivative) used. When using cellulose (method A) the molar ratio is preferably from 1.0 to 1.5; for products which are already etherified (method B) it is preferably from 0.1 to 1.0 mol of base per anhydroglucose unit.
The water content of the reaction mixture is preferably from 5 to 30, in particular from 10 to 20, mol of water per anhydroglucose unit.
After the suspension medium has been introduced as initial charge, the cellulose added and the batch rendered alkaline with the aqueous base, the batch is homogenized thoroughly and stirred without supply of heat, with cooling if desired, for preferably from 0.5 to 2 hours. The etherification reagents (epoxyalkanes, alkyl glycidyl ethers and/or alkyl halides and sulfonic acid derivatives) are subsequently added in unison or in succession. The batch is then brought preferably to a temperature in the range from 60 to 120xc2x0 C., with particular preference from 80 to 100xc2x0 C., and is heated for preferably from 2 to 6 hours. After cooling, it is neutralized with an acid, preferably hydrochloric acid, nitric acid and/or acetic acid, preferably to a pH of from 6 to 8. The suspension medium is removed by decantation or filtration. The crude cellulose mixed ether can be freed from the adhering byproducts, such as polyglycols, glycol ethers and salts, by extraction with aqueous alcohols or ketones having a preferred water content of from 10 to 50% by weight, especially isopropanol, ethanol and acetone. After drying under reduced pressure or at atmospheric pressure at from 50 to 120xc2x0 C., the desired cellulose mixed ether is obtained as a colorless or slightly yellowish powder.
If required, the degree of polymerization desired in accordance with the invention for the cellulose ether can be established prior to or during its preparation process by the addition of a peroxo compound, such as hydrogen peroxide, or a peroxodisulfate salt or other oxidizing agent, sodium chloride being one example. These methods of decreasing the molecular weight, and the respective industrial procedure, are prior art (T. M. Greenway in xe2x80x9cCellulosic Polymers, Blends and Compositesxe2x80x9d, ed. R. D. Gilbert, Carl Hanser Verlag, Munich, 1994, p. 178 ff.).
Suitable reaction apparatus for preparing the cellulose ether derivatives of the invention comprises, for example, stirred vessels, mixers and kneading apparatus. In principle it is possible to use any reaction apparatus which is customary for the preparation of cellulose derivatives having nonhydrophobic substituents and which allows sufficiently thorough mixing of the cellulose or water-soluble cellulose ether with the nonhydrophobic reagents.
The invention additionally provides for the use of water-soluble ionic cellulose ethers from the group of hydroxyalkylcelluloses which are substituted by on average from 0.001 to 1.0, preferably 0.001 to 0.2, alkyl group per anhydroglucose unit and carry from 0.01 to 0.4, preferably 0.01 to 0.15, particularly preferably 0.01 to 0.10 and particularly 0.01 to 0.09, sulfoalkyl group, preferably sulfoethyl group, per anhydroglucose unit, as protective colloids in connection with the preparation of aqueous polymer dispersions by means of free-radically initiated polymerization of ethylenically unsaturated monomers in aqueous emulsion, and provides an aqueous polymer dispersion prepared by free-radically initiated polymerization of ethylenically unsaturated monomers in aqueous emulsion in the presence of from 0.2 to 5.0% by weight, based on the total amount of the monomers used, of water-soluble ionic cellulose ethers from the group of hydroxyalkylcelluloses which are substituted by on average from 0.001 to 1.0 alkyl group per anhydroglucose unit and carry from 0.01 to 0.4 sulfoalkyl group per anhydroglucose unit.
The proportion of the cellulose ethers of the invention in connection with the preparation of such polymer dispersions is preferably from 0.2 to 5.0% by weight, and with particular preference from 0.3 to 1.0% by weight, based on the total amount of monomers used.
Suitable monomers are ethylenically unsaturated, free-radically polymerizable compounds which are themselves insoluble in water, examples being simple ethylenically unsaturated hydrocarbons having chain lengths of from 2 to 12 carbon atoms, preferably ethylene and propylene; esters having chain lengths of from 2 to 12 carbon atoms of acrylic, methacrylic, maleic, fumaric or itaconic acid, preferably ethyl, propyl and butyl esters; vinyl esters of unbranched and branched carboxylic acids having chain lengths of from 1 to 12 carbon atoms, especially vinyl acetate and Versatic acid vinyl esters; ethylenically unsaturated aromatic compounds, preferably styrene; ethylenically unsaturated aldehydes and ketones having 3 to 12 carbon atoms, preferably acrolein, methacrolein and methyl vinyl ketone, and halogenated ethylenically unsatured compounds, and example being vinyl chloride.
Particular preference is given to mixtures of the abovementioned monomers in which at least one component is a vinyl ester, preferably vinyl acetate. It is also possible to use mixtures of one or more of the monomers mentioned with hydrophilic monomers, for example, acrylonitrile, acrylic acid, methacrylic acid, itaconic acid or mixtures thereof.
An aqueous polymerization recipe in which water-soluble ionic cellulose ethers from the group of the hydroxyalkylcelluloses which are substituted by on average from 0.001 to 1.0 alkyl group per anhydroglucose unit and carry from 0.01 to 0.4 sulfoalkyl group per anhydroglucose unit are used as protective colloids preferably contains from 10 to 70% by weight, preferably from 30 to 60% by weight, of the abovementioned monomers plus from 0 to 10% by weight of one or more emulsifiers. Free-radical initiators customarily used are diazo compounds, redox initiators, organic or inorganic peroxo compounds, in amounts of from 0.1 to 3% by weight, preferably from 0.5 to 1% by weight, based on the total amount of the monomers. Further auxiliaries, examples being buffer substances or preservatives, can be added.
All components can be included together in the initial charge at the beginning of the reaction. in which case the monomer or monomer mixture is emulsified by stirring or by means of other mixing equipment. The polymerization process is started by raising the temperature. The temperatures required are dependent on the initiator system used and are between 40 and 120xc2x0 C. Following the onset of the reaction, cooling may also be necessary as a result of the exothermic nature of the reaction. The end of the reaction is discernible by abatement of the evolution of heat. In order to complete the reaction this is followed, optionally, by an after-reaction with external supply of heat. After cooling, auxiliaries for adjusting the pH, such as, for example, buffers, acids or bases, or for stabilization, such as preservatives, can be added. Optionally, the polymerization can also be started with a fraction, for example, from 10 to 20% by weight, of the amount of monomer and free-radical initiator, and further monomer and free-radical initiator can be added following the onset of the reaction, preferably in such a way that the desired polymerization temperature is controlled by means of the addition.
The dispersions obtained in accordance with the invention have the following characteristic properties:
Viscosity of the dispersions at low shear rate (1.0 sxe2x88x921):
for good processability and dispersion stability a viscosity of between 10,000 and 30,000 mPaxc2x7s, in particular from 15,000 to 25,000, is preferably desirable.
Viscosity of the dispersions at a high shear rate (xe2x89xa7250 sxe2x88x921):
for the dispersions to be readily conveyable, the viscosity at a high shear rate should preferably be  less than 450 mPaxc2x7s (250 sxe2x88x921), in particular from 200 to 420 mPaxc2x7s.
Average particle size of the dispersion:
The average particle size of the dispersion should preferably be from 200 to 300 nm (measured at a wavelength of 435 nm) in order to prevent unwanted settling of the dispersion (formation of serum).
Amounts of coagulum after filtration of the dispersion through a 100 xcexcm and 40 xcexcm sieve, expressed in mg per 1000 g of dispersion:
the dispersions have a coagulum content of  less than 200 mg/kg of dispersion with 100 xcexcm filtration and  less than 300 mg/kg of dispersion with 40 xcexcm filtration.
Water absorption of the dried polymer films:
The dispersion is poured onto a plate and dried to form a film. After treatment with water, the 1st water absorption (in % by weight of the weight of the polymer film itself) is determined via the weight increase. After drying again, the 2nd water absorption is determined. The 1st water absorption is generally greater than the 2nd water absorption, since the hydrophilic components (emulsifiers, protective colloid) are washed out during the 1st irrigation of the film. It should preferably be less than 25%, with particular preference between 5 and 20% by weight.
In addition to the performance parameters set out above, the grafting yield of the protective colloid used has an important part to play. A high grafting yield indicates high efficiency of the protective colloid. However, excessively high grafting yields lead to instances of crosslinking of the polymer particles, in conjunction with high coagulum contents and dilatent flow behavior of the dispersion, The grafting yield should preferably be between 15 and 30%, with particular preference between 20 and 25%.
The use of the hydrophobically modified, sulfoalkyl-containing hydroxyalkylcelluloses of the invention in connection with the preparation of vinyl dispersions has the advantage that the amount required is only half that of the conventional, commercial hydroxyethylcellulose, and that the dispersions which are prepared using the protective colloids employed in accordance with the invention are of superior quality. Dispersions which are prepared using alkyl-containing hydroxyethylcelluloses which are outside the degrees of substitution claimed are of significantly poorer quality (Comparative Examples).